Abstract : This report is divided into two main sections, both concerned with failure mechanisms in silicon semiconductors. The first part is dealing with investigations of two surface failure modes, namely, surface breakdown phenomena in oxide protected silicon devices, and influences of mobile surface ions on the characteristics of dilicon devices. A circuit is described which can be used for a light source under pulsed condition. Experiments are reported which demonstrate the existence of an Isolated Maximum of Influence on Breakdown. This IMIB-effect is investigated as a function of the geometrical position of the light spot as to the microplasma and as a function of the wavelength of the light. The possible influence of infrared light is discussed. The following major section of this report deals with the effects of mobile surface ions. The diodes are surrounded by gate rings. The influence of the gate bias on the distribution of the surface ions is investigated. Time effects of the surface ion distribution are observed. The second part of this report describes further investigations of the second breakdown phenomenon in silicon power transistors. A modification of the pulse circuit is described which allows the generation of high voltage, low current or high current, low voltage pulses. The two pulse modes are used to study their influence on the thermal instability and second breakdown behavior of experimental power transistors and simplified transistor structures. (Author)